Plain and simple: driving while intoxicated isn’t smart — to put it nicely. But while that may be easy to understand while sober, it can be less obvious for individuals to recognize and understand when drunk. It can even be more frustrating to convince a friend or loved one of how drunk they are, that despite what they may think they don’t actually drive better when drunk, and get them to hand over their keys in the process.

Thankfully the BreathalEyes is here to help. While there may very well be an app for this or that, the BreathalEyes app, apart from being a very clever play on words, is probably more useful and potentially lifesaving than a great deal of others available.

For anyone who has ever been pulled over and given a field sobriety test, or watched an episode of Cops, one of the methods employed by law enforcement is to test for Horizontal Gaze Nystagmus (HGN) before they slap some steel around your wrists and haul you off.

But what exactly is HGN? No it isn’t some home and garden channel, HGN is the involuntary jerk and twitches your eyes make when the iris reaches an angle of about 45 degrees while your head remains forward. Cops generally administer such tests and check for HGN in the field with a pen or some variation of that.

What the BreathalEyes does is really to help your friends (and in doing so help you) conduct your own HGN test before you make the poor decision of driving drunk. The app utilizes the iPhone 4’s camera to detect the subjects eye movement by asking the person to look as far as possible in one direction while the camera monitors for any erratic or involuntary eye movement. After that, the app uses the collected data to gauge the blood alcohol level of the person being tested. To ensure a level of accuracy, the developers of BreathalEyes have conducted tests alongside an actual breathalyzer and found it to carry an accuracy range of 0.02% – 0.17% (+/- .02%) BAC.

Of course BreathalEyes isn’t perfect and its creators have placed numerous disclaimers to save them from any legal trouble that may arise. Still, the slight inconvenience of administering the test and purchasing the $0.99 the app costs is a small price to pay given the much steeper price one might later pay for driving drunk.

The BreathalEyes app is available now on the iPhone 4 and iPhone 4S for $0.99 with an Android version in the works for later this year.